Battery-backed RAM
Also called SRAM.
Battery powered RAM (made non-volatile) used to store cab related informations and scores tables in MVS systems.
The CD systems use it to store game saves instead of using memory cards.
BIOS functions are used to handle the specific save format, although read/writes can be freely done across the whole memory by the game's 68k program.
Battery replacement
NiMh/NiCd, CR2032, charging resistor removal...